FFT

Discussion in 'Homework Help' started by hkwj88, Mar 27, 2013.

1. hkwj88 Thread Starter New Member

Mar 27, 2013
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I'm having problem with question 3a. From my understanding, this FFT shows 3 Harmonics with a fundamental frequency of 1Mhz. Am i wrong to say that?

Could anyone help me? My Prof doesn't know anything too.

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2. WBahn Moderator

Mar 31, 2012
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If the signal consists of three harmonics with a fundamental frequency of 1MHz, the you would have three signals at k_i*1MHz where k_i are three different strictly positive integers.

Does that look like what you've got?

3. panic mode Senior Member

Oct 10, 2011
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are you sure you see 3 peaks? i see 4...

4. WBahn Moderator

Mar 31, 2012
23,566
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I'm assuming he's simply discounting the DC component, but that was my first thought, too.

5. vk6zgo Active Member

Jul 21, 2012
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Ignore the 0Hz peak.

What do you notice about the frequencies of the other 3 peaks?

Remember the definition of harmonics--does that make any sense for a fundamental of 1MHz?

6. JoeJester AAC Fanatic!

Apr 26, 2005
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I'm thinking the OP did not read the question at all. If the primary frequency was 1 MHz, and they want to notch out 10kHz, what is all this discussion about harmonics?

It appears the signal of interest is being interferred with by the 10kHz signal, but there was no information about the notch specifications. I know in some systems, signals less than -60 dB from the signal of interest can cause unwanted issues, so I suspect a neg 60 dB notch would be in order.

I'm sure your professor will be thrilled to learn you commented on their skills.

7. WBahn Moderator

Mar 31, 2012
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I have to admit that I didn't look at the information in Part B in an effort to try to figure out what was being asked for in Part A. Any problem that you have to do that for is a poorly constructed problem, IMNSHO, but when a question is ambiguous are hard to understand, looking at the other parts (and sometimes even other problems) can shed some light.

8. panic mode Senior Member

Oct 10, 2011
1,645
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i doubt he is coming back... probably figured to stick with Yahoo Answers ;-)

9. WBahn Moderator

Mar 31, 2012
23,566
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Yep. Another one post wonder.

Boy, and speaking of Yahoo Answers (and similar sites). I've found a few decent answers to things at those places, but most of it strikes me as pure crap.

10. panic mode Senior Member

Oct 10, 2011
1,645
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the problem is with some students not wanting to fix root of the problem such as bad approach or misunderstanding, they would rather just have the answer served on a silver platter. moreover they miss out on communicating things is a efficient manner (like asking question properly). then they blame professor. while i've met few professors that didn't quite dazzle, problem was hardly that "they didn't know anything", more like they lacked skill to communicate and pass on their knowledge (illegible handwriting, incomplete sentences, poor planing, not approachable, etc.).

Speaking of Yahoo Answers and similar, most of the questions are incomplete or even incoherent. moderating team spends way more time removing replies than anything else. maybe it is time for site that does similar job differently, perhaps having requirements to complete certain fields and have review/approval before they can be actually hosted (at least for new members). hmmm... keep dreaming..

11. WBahn Moderator

Mar 31, 2012
23,566
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Yep, keep dreaming.

It's bad enough with sites like Yahoo Answers and such. But what really gets to me is the number of sites where some company designs something, be it software or hardware, and then produces hardly any documentation at all because they have a "knowledge base" where users can "collectively construct" documentation that is "for users by users". Sheesh.

12. hkwj88 Thread Starter New Member

Mar 27, 2013
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i think i'm confused by all the comments, so would i be right to say that 0hz is the Dc component?

13. panic mode Senior Member

Oct 10, 2011
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459
wow, you are back...

yes 0Hz is the DC components. and other spikes are signals of given frequencies and amplitudes. to have harmonics, you would need to see peaks that are integer multiples of the fundamental frequency. in general amplitude of higher harmonics quickly decays so it 9th harmonic should have lower amplitude than 2nd or 3rd. 15th or so would probably be invisible. suppose your 100Hz peak is fundamental frequency, if it had harmonics they would be near (2x,3x,4x,... yields 200Hz, 300, 400, 500,...). getting to 100x is not gona happen - specially when lower harmonics are missing. therefore 10kHz is not a harmonic, it is another fundamental frequency. use same reasoning and you get to 1MHz peak. so none of them are harmonics, you have DC component and three fundamental frequencies. we can tell the frequencies and amplitude but not phase. that's it.

14. hkwj88 Thread Starter New Member

Mar 27, 2013
3
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thanks for your enlightenment. was busy studying for exam. pardon me for my late reply.